Distortion

Macro lenses are reknowned for minimal distortion and the 150mm f/2.8 APO EX is no
exception here - the degree is absolutely negligible.

Vignetting

On an APS-C DSLR such as the Canon EOS 350D vignetting is no significant issue. At infinity
focus and f/2.8 the problem may be visible in very critical scenes but this should be
a very rare event. From f/4 onwards there's basically no vignetting anymore. At close
focus distances the issue is even less pronounced.

MTF (resolution)

In the lab the sample lens exhibited a very high potential. You may notice the rather
diplomatic summary here - more on this later. The MTF curve indicates a very high
performance straight from f/2.8 with little difference between the center and the
corners. The quality peaks at f/5.6 where the lens scratches or even exceeds the
resolution limits of the EOS 350D. Beyond that diffraction effects are limiting the
performance - this cannot be avoided and it is therefore a normal behaviour. Ok, that
was the good part ...

The bad message is that the sample lens showed a rather extreme degree of decentering
resulting in a very pronounced and disturbing drop in performance towards the upper
left corner which was visible up to f/5.6 (see also the first sample image in the
2nd row below - due to the vertical layout the decentering effect is visible to the
lower left here).
At f/2.8 the resolution was measured with 640 LW/PH here which is almost just
a third of the lower right image corner which was chosen instead for the MTF chart below.
This is probably an isolated incident with this specific sample lens but frankly it is
quite unacceptable for a lens in this price class.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic aberrations

Typical for most fix-focals chromatic aberrations (color shadows at harsh contrast transitions)
are exceptionally well controlled and no issue to worry about.